Color proofing is a process used by the printing industry to simulate proofs generated on a press run. By using color proofing the printing industry saves time and money simulating how the press will look before the costly press run is performed. The advantage of a color proof is that it is a representation of an ideal press run. The color proof should reflect exactly what the printing industry would like to see coming off a press. The press is continually adjusted to match the color of the proof. The color proof therefore needs to be precise color, reproducible from proofer to proofer, and pre-press shop to pre-press shop. Proofs that exhibit color variation are deemed unacceptable.
One commercially available image processing apparatus, which is depicted in commonly assigned U.S. Pat. No. 5,428,371, is an image processing apparatus having half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media by transferring dye from a sheet of dye donor material to the thermal print media by applying thermal energy to the dye donor material, to transfer dye to the thermal print media, thereby forming an intended image. This image processing apparatus is comprised generally of a material supply assembly or carousel, lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, and vacuum imaging drum), and thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises feeding a sheet of thermal media from the media roll to the vacuum drum, partially wrapped around the drum, cut to length, then wrapped fully around the drum. A length of dye donor from a roll form is similarly transported to the drum, partially wrapped around the drum, cut to a desired length, then fully wrapped around the vacuum drum. The dye donor material is wrapped around the vacuum imaging drum, such that it is superposed in registration with the thermal print media. The translation drive, part of the scanning subsystem, traverses the printhead and translation stage member axially along the vacuum imaging drum in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.
The printhead includes a plurality of laser diodes which are coupled to the printhead by fiber optic cables which can be individually modulated to supply energy to selected areas of the donor in accordance with an information signal. The printhead includes a plurality of optical fibers coupled to the laser diodes at one end and at the other end to a fiber optic array within the printhead. The printhead moves relative to the longitudinal axis of the vacuum imaging drum and dye is transferred to the thermal print media as the radiation, transferred from the laser diodes by the optical fibers to the printhead to the dye donor material, is converted to thermal energy in the dye donor material.
Color variation is typically a result of variation of the individual color density used to define to desired color. There are many factors that influence the density variation of a color in a proof. One factor that influences proof density uniformity is the focus position of a printhead. Focus is defined by the distance between the lens and the imaging plane along the optical axis. There exists an optimum distance between the imaging plane and the end of the lens for maximum density transfer. As the focus position varies from the optimum position density will also vary. The relationship between focus position and density is often parabolic in nature. As the actual focus position moves away from the theoretical best focus position the density will vary exponentially.
Several factors contribute to focus related errors. Some existing color proofs, for example U.S. Pat. No. 5,428,371, use a translation system to move the printhead across the imaging plane. Without perfect alignment of translation components an optical axis distance error will be introduced. Thermal media used on color proofing systems have some level of thickness variability. Thickness variation is typically highest in the same direction as the proofer translation. Choices of hardware to position the printhead in the optical axis direction influence the ability to place the printhead to a desired position. Environmental effects, specifically changes in temperature, can introduce optical axis distance changes and instability of a focus position.
Some early digital proofers, such as U.S. Pat. No. 5,268,708 utilized an auto focus system to detect changes in optical axis distance due to drum and rod alignment and media thickness changes. This system used a reflected laser to measure distance changes. Disadvantages of this system include the presence of unwanted noise which did not allow the system to work properly. The system was also damaged frequently and was costly to replace. This lead many service engineers to simply disconnect the auto-focus device, essentially leaving the system as a fixed-focus system.
One color proofing system uses a fixed focus design. The system is designed to allow acceptable print uniformity at a optimum focus position. The best focus patch is determined by selection of patches for a course adjustment proof and then a fine adjustment proof. The course and fine focus proofs are written in a similar manner. A single linear ramp of focus positions is imaged across the translation direction. The best patch is defined as the patch with maximum density. This patch selection method leads to several disadvantages.
Using a method of printing patches to determine the best focus position introduces errors due to print noise as well as errors due to selecting a single patch, rather than interpreting between patches. The best focus position is essentially limited in accuracy to the focus step size of the fine adjustment proof. Print noise comes from various sources. One error as previously started is changes in focus position. Another error is coating quality of thermal media, both receivers and donors. Coating non-uniformities typically are seen in the same direction as the translation system of the proofer. Errors arrive from optical noise and fiber optic blooming as a result of moving fiber optic cables during the printing process as well as variability within lasers and laser power supplies. A densitometer or spectrophotometer used to measure patches introduces random density errors. Without using statistical methods to find the optimum focus position results will exhibit unacceptable amounts of error.